A major contribution to the cost for a single base station is the cost of the antenna structure used to transmit/receive radio frequency signals from/to the base station. In order to reduce the cost of establishing a communication network, it is becoming more and more common to share or co-site e.g. antenna structures between multiple base stations, recognizing the different communication systems will transmit/receive on different radio frequencies.
Despite the filtering circuitry of individual base stations and transmit amplifier specifications which limit acceptable spurious noise levels at frequencies outside the allocated block of spectrum, the frequency bandwidths allocated to different communication systems may be near enough that the conventional implemented filtering performed by each base station will be insufficient to prevent interference between the communication signals of each wireless system in a shared antenna environment. Additionally, the physical connection of transmission lines from multiple base stations at a common connection point will generally cause considerable power loss i.e. insertion loss attributable to the transmit/receive signal of one system feeding into the transmission line of the second system. Such insertion loss will require increased power and/or a higher gain antenna structure to achieve acceptable signal-to-noise characteristics.
One specific problem for sharing one antenna is the problem of separating the signals to be transmitted from the antenna from the signals received at the antenna. Another problem is how to separate the respective signals to and from the respective co-sited base stations.
According to prior art this can be achieved with the use of so called duplex filters. Duplex filters combine the transmitter and receiver signals of a specific frequency band into a common signal. The filters are usually built into the base stations, but can also be used as external components in the antenna system. Such a filter arrangement typically has a first band pass filter which passes frequencies in the transmit band and a second band pass filter which passes frequencies in the receive band.
Also, the physical connection of transmission lines from multiple base stations to a common connection point will cause considerable power loss, thus requiring increased power and/or a higher gain antenna structure to achieve acceptable signal-to-noise characteristics. In prior art this has been achieved by supplying the transmitted signals to a multi carrier power amplifier structure, enabling amplification of the individual transmitted signals.
Therefore, there is a need for solutions for enabling co-siting or feeder sharing of base stations while maintaining a tolerable level of insertion loss and a tolerable accuracy in the transmit power.